1. Field of the Invention
This invention relates to an ultrasonic transmitter/receiver, more particularly to an improved ultrasonic transmitter/receiver for detecting and measuring the movement of a moving member within a subject under examination utilized in a two dimensional doppler device.
2. Description of the Prior Art
There are known ultrasonic transmitter/receivers which detect and measure the velocity of a moving member within a subject under examination by transmitting an ultrasonic beam into the subject and then receiving the same beam after reflection by the moving member. These transmitter/receivers are widely used for noninvasively measuring the velocity of movement of a body organ such as the heart, of blood flowing in blood vessels or of other body fluids.
The operating principle of such an ultrasonic transmitter/receiver is schematically illustrated in FIG. 1. A probe 10 is used to transmit an ultrasonic beam 100 into a subject 12 as pulses of a given fixed pulse-repetition frequency and to receive the pulses after they are reflected from some member 14 within the subject. Then, using the moving target indication (MTI) method, correlated comparison method, phase comparison method or some other known method, signal processing is carried out in each cycle to compare the signal resulting from the transmission and reception of the ultrasonic pulse in the current cycle with that obtained in the preceding cycle.
With this arrangement, when there is no moving member within the subject 12, the signal 200 (FIG. 2) obtained by transmitting and receiving the ultrasonic beam 100 during the current cycle and the signal 200' obtained by transmitting and receiving the ultrasonic beam 100' in the preceding cycle are of precisely the same amplitude and phase. As a result, no output signal is obtained by subjecting these two signals to comparative processing. In other words, the level of the comparison signal 300 is zero.
On the other hand, when a moving member exists within the subject, the signal 200 obtained by transmitting and receiving the beam 100 in the current cycle and the signal obtained by transmitting and receiving the beam 100' in the preceding cycle differ in both amplitude and phase, as shown in FIG. 3. Therefore, when these signals are subjected to comparative processing, there is obtained a comparison signal 300 corresponding to the amount of movement by the moving member 14.
With this ultrasonic transmitter/receiver then it is possible to detect and measure the velocity of the moving member 14 within the subject on the basis of the comparison signal 300 obtained by comparing the signal obtained by transmitting and receiving the beam 100 in the current cycle with that obtained by transmitting and receiving the beam 100' in the preceding cycle.
The conventional ultrasonic transmitter/receiver has, however, been able to carry out accurate detection and measurement with respect to the moving member 14 only when the ultrasonic beam 100 is transmitted into and received from the subject 12 in one fixed direction. When the ultrasonic beam 100 is made to scan the subject 12 by changing its direction at a predetermined velocity, the signal 200 will change in phase and amplitude even when there is no moving member present within the subject 12. As a result, scanning noise will arise in the comparison signal 300.
This will be clear from an analysis of the acoustic model in FIG. 4, which shows a stationary body 16 being scanned by the ultrasonic beam 100 in the direction indicated by the arrow. In this case, as shown in FIG. 5, the signal 200 obtained by transmitting/receiving the ultrasonic beam 100 in the current cycle and the signal 200' obtained by transmitting/receiving the ultrasonic beam 100' in the preceding cycle differ from each other in both phase and amplitude. As a consequence, the comparison signal 300 obtained by comparative processing of the signals 200 and 200' contains a scanning noise component 400.
From this it will be understood that with the conventional device, when the ultrasonic beam 100 is made to scan by changing its direction at a given velocity, scanning noise arises in the comparison signal 300, making it impossible to detect and measure the velocity of movement of a moving member within the subject 12 with high accuracy.
Because of this problem, one technique used in conventional devices is to set the scanning angle .theta. between adjacent ultrasonic beam pulses 100, 100' transmitted into the subject 12 at a very small value so as to minimize the scanning noise component 400. Use of this method, however, tends to result in excessively fine scanning which makes it impossible to scan the ultrasonic beam 100 at high speed.
Another practice resorted to is to repeat the transmission/reception of the ultrasonic beam 100 a number of times in one fixed direction in order to obtain a like number of signals 200 from which there can be derived a comparison signal 300 free from scanning noise, and then to repeat this operation at successive scanning positions so as to carry out scanning in a stepwise manner. One problem with this method is that it can be applied only to electronic-scanning type ultrasonic transmitter/receivers which carry out quantized scanning and cannot be applied to mechanical-scanning type ultrasonic transmitter/receivers which carry out continuous scanning. Another is that devices using this method are unable to scan the ultrasonic beam 100 at high speed.